The present invention relates generally to coating systems for turbine blades and shrouds for gas turbine engines and more particularly, to coating systems having an abrasive coating on the turbine blade tip and a complementary abradable coating on the inner surface of the turbine shroud.
Gas turbine engines typically include a variety of rotary seal systems to maintain differential working pressures that are critical to engine performance. One common type of seal system includes a rotating blade positioned in a rub relationship with the inner surface of a shroud. With the operation of a gas turbine engine, blade tip wear during rubs with the shroud along with blade tip oxidation can reduce blade tip height and increase the blade tip to shroud clearance. Reduced blade tip clearance increases turbine performance and performance retention during the service of the gas turbine engine, resulting in a decrease in the expense of operation and maintenance of the engine.
Several rotary seal systems to minimize the blade tip to shroud clearance have been described in the prior art. The prior art systems basically have blades with ceramic coated tips that have the ability to abrade the inner surface of the shroud. One system, disclosed in U.S. Pat. No. 5,059,095 has a blade with a ceramic blade tip layer where the layer consists of aluminum oxide and zirconia-based oxide. U.S. Pat. No. 6,190,124 discloses a similar system having a blade with an abrasive tip that is harder than an abradable inner shroud surface. The blade tip has a metal bond coat, an aluminum oxide layer disposed on the metal bond coat and a zirconium oxide abrasive coat disposed on the aluminum oxide layer where the zirconium oxide abrasive coat has a columnar structure. However, while these rotary seal systems are an improvement over a blade and shroud with no abrasive or abradable coatings, respectively, they focus on an abrading blade tip coat but not on a corresponding inner shroud surface compatible with the abrading blade tip coat that would minimize the rubbing friction during engine operation. Such friction may result in bending stresses that may crack or overload the blade to failure.
As can be seen, there is a need for a rotary seal system for gas turbine engines having a blade with an abrading tip coating and a shroud with an abradable inner surface that is compatible with the abrading coating of the blade tip such that the friction of rubbing between the blade tip and the inner surface of the shroud is minimal. Such a rotary seal system should also maintain a minimum clearance between the blade tip and the inner surface of the shroud.